Heretical mode on third gen Prius, what do we know?

If you keep the ICE rpm at the shown speed, the MG1 will stay at 0 rpm.

Ken@Japan

 

A motor doesn't have to be spinning appreciably fast to exert
torque.
.
In any HSD type system, MG1's job during highway-cruise is to
*counter* any leftover torque from the engine that isn't going
to the wheels. Its speed while doing so is quite irrelevant,
forward or backward, and is only a *side effect* of the commanded
throttle and RPM from the engine. Nothing special happens when
it crosses through 0 RPM and reverses direction either way.
It's still exerting that countertorque.
.
I've explained this before, but people keep missing it.
.
_H*

 

It is only the difference in energy flow, between normal and energy re-circulate that changes:

At low speeds, MG1's counter torque from generating power is sufficient to handle the 28% counter torque needed to force the ICE torque to pass through the power split device and on to the wheels. The excess power from MG1 recombines at MG2 and on to the wheels.

At higher speeds, MG1 needs power to provide the 28% counter torque:

But at higher speeds, MG2 can generate the power needed to turn MG1. The battery simply handle the transients.

This is what a transition event looks like:


MG1 speed is as Hobbit points out, unimportant. It is the energy flow direction from MG1-to-MG2 and later, MG2-to-MG1, that is interesting. This reversal of energy flow is critical ... and it made my head hurt to figure it out.

The original definition of "heretical" mode was based upon MG1 rpm direction. The important definition is which way the energy flows in the dance between MG1, MG2, traction battery and ICE.

Bob Wilson

 

In fact it doesn't have to be spinning at all to exert torque.

Tom

 

Bob,

Thanks for the data. The graph shows very interesting data around when MG1 goes to zero. 8 kW is drawn from the HV pack... is it needed to reverse the MG1 direction? I can't say for sure because ICE power dropped dramatically at the same time as well. I am guessing the power from HV pack is drawn to compensate for the ICE.

If it takes a lot of energy to reverse MG1 direction, it would be wise to avoid it. Lead footed driver that fluctuate the pedal would cause HSD to consume extra energy reversing MG1 direction very often. This may be a technique Top Gear used at the race track to bring down the MPG in comparison with the M5.

One question: Toward the end of the graph, why did MG1, MG2, HV and ICE goes to zero except the MPH?

 

Your point is taken. MG1 in negative RPM allows ICE to spin slower at a given speed. GenIII 1.8L ICE with more torque would probably benefit more from this.

As a guess in my previous post, switching between H mode and the regular mode will consume more energy. Sudden acceleration will cause this mode switch. Climbing a hill may as well, if the driver suddenly notice the speed drop and try to accelerate while climbing. Planning ahead by gradually pressing the pedal prior (keeping ICE RPM below the threshold per Ken's table) to the hill will prevent the mode switch.

High speed driving (70-75 MPH) requires more power to maintain speed as well. A slight acceleration at those speed may trigger the mode change.

What are your thoughts?

 

This is the part that tends to bring on the headache ... it just changes from being a generator making power to a motor, taking power. It happens in an instant.

I probably shifted into "N". That turns off MG1 and MG2 and from that point on, no power is going to or from the wheels.

Please don't think in terms of MG1 rpm and direction as MG1 torque is all that counts. Torque is the mechanical equivalent of thinking of a circuit in terms of current, not voltage (unless there is a resistance involved.)

Bob Wilson

 

For the non electrical engineers reading this, think of this analogy:

We have two bicyclists riding two bicycles, with bicycle A in front of bicycle B. A broom handle is fastened between the two bicycles, so they always move at the same speed. At first bicyclist B is feeling very strong, so he pedals hard and pushes bicycle A. After a bit, he starts feeling lazy, so he stops pedaling. Now bicyclist A is towing bicyclist B.

The speed is the same in both cases, only in one case bicyclist B adds power, and in the other he becomes a drag. This is exactly what happens when MG1 switches from being motor to a generator. It can tootle along at the same speed, in one case adding power, the next sucking up power.

Tom

 

Bob, MG1 did reverse direction from your data.

@304th second: ICE = 15kW (~1,230 RPM), Car = 22 MPH (MG2=1,300 RPM), MG1 = +1,000 RPM
@317th second: ICE = 23kW (~1,909 RPM), Car = 42 MPH (MG2=2,500 RPM), MG1 = +300 RPM
@319th second: ICE = 8kW (~990 RPM), Car = 43 MPH (MG2=2,560 RPM), MG1 = -2,200 RPM

We began to see electricity consumption from the HV pack precisely at 319th second. 2kW was used by the MG2 per the graph. I think 6kW was used by MG1 to bring itself up to -2,200 RPM (from +300 RPM). The reversal of MG1 brings down the ICE to the "idle" 990 RPM.

 

I don't remember uploading the spreadsheet but I may have in the past. I'll check and if I still have the data, I'll try to upload the spreadsheet. The reason is I'm not sure we can get "8kW @990 RPM." The raw data would have had the actual, measured rpms and torque.

Bob Wilson

 

So at highway speeds, MG2 is constantly generating power for MG1 to run heretical, correct?

OT: if MG2 is air-cooled, is it possible for it to be "too cold" for best efficiency? Because if that is the case, it might explain mpg losses at highway speeds in below-freezing temps.

 

Bob,

I was getting the data from your graph. I converted kW to ICE RPM using the ideal 82 lbs-ft torque, which is why I indicated approximate. The 8kW using that torque came out to about 560 RPM which didn't make sense because Prius never go between 0 and 990 RPM -- so I used the most common idling RPM.

If you have the data in spreadsheet, it would be awesome.

 

Yes, but variable. MG2 is in the transaxle together with MG1 and both are liquid cooled.

For your GenIII, the heat from the exhaust is also used to keep the ICE and transaxle hot.

 

I could have sworn that someone posted that MG2 is air-cooled - was that the case for the Gen II or something?

I wonder if Scanguage can monitor the transaxle fluid temps - could be that the exhaust isn't hot enough while cruising to offset the extra-cold air flowing through/under the car at high speed...

 

The transaxle is both air and liquid cooled. It shares the inverter coolant loop but the large surface area also dumps a lot of heat. Internally, the transaxle oil also carries heat around but there is no separate oil cooler (or heater!)

Bob Wilson

 

Sorry,

I've not found the original file for the detailed heretical mode transition. But I think this one will do for now. This is one of my smaller files but that is another story.

Load it up and trim down to the areas of interest.

NOTE: I found the original file and in the interest of keeping space usage down will replace the 1.6 MB file with the original.

Bob Wilson

 

Thanks Bob. The data was captured with NHW11 correct?

 

Bob,

I made a graph of RPMs of ICE, MG1 and MG2 over time (in seconds). MG1 reversed direction 13 times in the first 3 minutes.

 

My data comes from the NHW11 but given similar instrumentation, the similar data can be collected from just about any Prius. But after taking a nap, I remembered where I'd left the orginal data:

Yahoogroup "Prius Technical Stuff"
​

Anyone who is serious about finding out how a Prius works would be well advised to go there and use the message search to look for information. The files, photos and supporting files are an exceptionally rich source of historical data. I joined in 2005 but this group started in 2000. Anyway, that is where I'd left the data file from the original graph.

Enjoy!
Bob Wilson